Specimens referred to Osteopygis (Late Cretaceous-Paleocene, North America) represent a chimera, a polyphyletic mixture of taxa. The holotype of Osteopygis (AMNH 1485) and more complete referred postcranial specimens resemble non-marine stem cryptodires (“macrobacnids”). Because the skull material historically referred to Osteopygis shares synapomorphies with chelomid sea turtles, all current workers accept Osteopygis as a stem-cheloniid sea turtle. Multiple lines of evidence combine to support the hypothesis that sea turtle cranial material is not attributable to Osteopygis. These lines of evidence include: phylogenetic hypotheses of character evolution, the tenuous historical attribution of specimens, and the taphonomy of the Hornerstown Formation. The name-bearing Osteopygis material and referred posterania are best considered Eucryptodira incertae sedis (cf. “Macrobaenidae”). The cranial specimens formerly assigned to the Osteopyginae now are restricted to the clade Euclastes and those referred to Osteopygis emarginatus are here referred to Euclastes wielandi (comb. nov.). The ‘decapitation’ of Osteopygis reconciles morphological trends within stein cheloniids.